This application relates generally to positive grids for lead acid batteries. More specifically, this application relates to lead alloy coatings for the positive grids of lead acid batteries and methods of using such coatings.
Lead-acid batteries are multi-cell structures with each cell containing a positive plate or electrode, a negative plate or electrode, and an electrolyte. Each plate consists of a grid of expanded metal having a layer of electrochemically active material formed thereon.
The structure of the positive plate, namely the grid structure and grid material, affects the life and current generating efficiency of the battery. The grid is expanded from a strip of lead or lead alloy. For example, U.S. Pat. Nos. 3,853,626 and 3,945,097 to Daniels et al. describe exemplary methods and equipment for making such expanded grids and are herein incorporated by reference in their entirety.
The active material is applied to the grid after expansion. The active material on positive plates is typically lead dioxide (PbO2), while the active material on the negative plates is typically sponge lead. Normally, a precursor to the lead dioxide is applied to the grid to make the positive plate. The precursor is then electrochemically oxidized to form lead dioxide.
The positive plate affects the life and current generating efficiency of the battery because they are subjected to severe cycling between oxidizing and deoxidizing reactions of the active material. Thus, grids of positive plates not only provide structural support for the active material, but also collect the current (energy) from the active material and transmit the current to lugs extending from the grid.
The cycling of positive plates leads to corrosion between the interface of the active material and the underlying grid material, know as the corrosion layer. Moreover, the positive plates expand and contract during the cycling. The combination of expansion, contraction, oxidizing reactions, and deoxidizing reactions limits the life of the positive plate, especially at elevated temperatures. Referring to FIG. 1, a prior art positive battery plate 10 is illustrated. After exposure of battery plate 10 to cycling described above, active materials 12, in the form of lead dioxide 14, exfoliates or separates from grid material 16. The cycling causes stress cracks 18 to form in active material 12 resulting in a loss of conductivity between grid 16 and the active material 12.
Surface properties of grid material 16 are often opposite the bulk properties necessary in the grid. Typically, processes and materials that strengthen the bulk properties of grid material 16 (e.g., wrought materials) damage the surface properties and lead to breaks in conductivity. Conversely, processes and materials that provide surface properties resistance to conductivity losses due to cracks (e.g., cast materials) provide little or no strength for the grid.
Conductivity losses have been abated through the use of antimony based coatings. However, antimony can cause problems in battery performance. For example during the cycling of the battery, the antimony gases and requires venting of the battery. Vented batteries have a high water loss and, thus, are typically not maintenance free batteries.
A lead alloy coating for a positive grid of a lead acid battery is provided. The lead alloy coating includes a tin content of at least about 0.1%, but not more than about 3%; and a residual lead content. The lead alloy coating optionally includes a calcium content of at least about 0.01%, but not more than about 0.1%, with or without a silver content of at least about 0.01%, but not more than about 0.1%. Alternatively, the lead alloy coating optionally includes a barium content of at least about 0.01%, but not more than about 0.1%, with or without a silver content of at least about 0.01%, but not more than about 0.1%.
A coated positive grid for a lead acid battery is provided. The coated positive grid includes a wrought lead or lead alloy strip and a cast lead alloy coating. The strip has a first surface with the coating disposed thereon. The cast lead alloy coating is selected from the group consisting of binary lead-tin alloys, ternary lead-calcium-tin alloys, quaternary lead-calcium-tin-silver alloys, ternary lead-barium-tin alloys, and quaternary lead-barium-tin-silver alloys. Moreover, the strip has a linear elongated grain structure parallel to the first surface, and the cast lead alloy coating has a random grain structure. The linear elongated grain structure provides mechanical strength to the strip, while the random grain structure mitigates conductivity losses caused by cracks in the coated positive grid.
A method of coating a positive battery grid for a lead acid battery is provided. The method includes providing a first layer and a second layer to a surface coating process, and coating a surface of the first layer with the second layer to form a coated strip. The first layer is a wrought lead or lead alloy material. The second layer is a cast lead alloy material selected from the group consisting of binary lead-tin alloys, ternary lead-calcium-tin alloys, quaternary lead-calcium-tin-silver alloys, ternary lead-barium-tin alloys, quaternary lead-barium-tin-silver alloys, and quaternary lead-calcium-tin-barium alloys.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.